Oil-Based and Bio-Derived Thermoplastic Polymer Blends and Composites 247
theories [23, 24] provide definition of훿depending on the energy of total evaporation
훿=
√
ΔHvap
V
in whichΔHvapis the energy of evaporation andVis the molar volume of the solvent.
This훿expression is valid only for regular solution (aliphatic and nonpolar solvents).
Hansen [25] extends훿proposing a multidimensional solubility parameter
훿=
√
훿^2 D+훿^2 H+훿P^2
where훿Drefers to nonspecific intermolecular interaction related to dispersion forces,
훿Hrefers to specific intermolecular interaction (such as hydrogen bonding, acid–base
interaction) and훿Prefers to dipole–dipole forces.
One of the parameters that govern miscibility between polymers is the interfacial
tension: the higher the interfacial tension, the higher the immiscibility between
polymers [26].
Having high interfacial tension leads to particle formation, which could coalesce,
decreasing mechanical properties. The general rule to achieve good blends is, like
chemists say, ‘similar dissolves similar’. Polymers with similar polarity are easier to
blend than polymers with different polarities. Other important features are as follows:
- Specific group attractions: polymers with groups that could provide bonds or
attraction between polymer chains are easier to blend.
- Molecular weight: polymers with low molecular weight are easier to mix, facilitating
miscibility. As mentioned earlier, polymers with similar molecular weight are more
miscible.
- Ratio: the mutual amount of polymers affects the possibility of mixing. A low amount
of polymer could be soluble in another polymer, while a higher amount could not.
- Crystallinity: generally, if polymers in a blend crystallize, they will provide different
crystalline phase, increasing the number of phases present. As a consequence, the
mixing between polymers will occur with more difficulty.
During last decades, many studies have been conducted in order to reduce the
amount of non-biodegradable charge. Thipmanee and Sane [27] analysed low-density
polyethylene (LLDP)/TPS blend with zeolite 5A nanoparticles (Z) (Table 8.7), trying to
improve the compatibility between the polymers.
The addition of the zeolite 5A allows obtaining both good mechanical and thermal
properties of LLDPE/TPS blend. Zeolite 5A increases both tensile strength and
elongation at break (Figure 8.4) and decreases both glass transition temperature and
crystalline temperature (Table 8.8). Usually, zeolite increasesTcacting as a nucleating
agent. In this study, zeolite allows an improvement in the miscibility of LLDPE/TPS
blend; as a consequence, starch molecules depress the crystallization of PE during
cooling. Decreases inTccould be considered as a proof of zeolite work as coupling
agent between LLDPE and TPS.
Because of their great importance and multitude of applications, PLA blends are
deeply studied nowadays, in order to reduce PLA brittleness. Hence, soft and tough
polymers can be blended to PLA, improving their properties and their applications. At
the same time, the production of biodegradable blends with traditional polyolefins is
also possible.
